Philosophically, it would seem that if you already have enough gain in your system, a passive should just pass on an exact copy of the magnificent sound produced by your source components attenuated down to the intensity of your desires. While it ain't necessarily that simple, I am finding that passive linestages can be very, very good in practice.
The theoretical problem with passives is that they can give a frequency response to your system which depends on their setting. For example, at low volumes they might be great, but as the setting is increased they might cause the system to attenuate the higher frequencies. This can be due to the interaction of the passive's output impedance with the capacitance of the interconnects and the input impedance of the amplifier. The output impedance of the source can also play a role. So, a passive won't necessarily work well in every system. (Note: There are many schemes to get around these effects and if you do a search of the 'Gon archives or the Audio Asylum a wealth of information will come up.)
My experience is similar to what's posted above. I am going through the process of comparing a resistive passive with active linestages. The resistive passive (ladder) is my reference.
The preliminary conclusion in my (all tube) system is that the passive is very tough (but not impossible, so far) to beat. I have it up against a CJ Premiere 16 Series II and a Nagra PL-P. It might be rolling off the very high frequencies at some settings, but not by much and possibly this can be controlled by system matching the impedance of the passive, interconnects, and amplifier input.
The differences in performance between the above three linestages can be quite dependent on upstream (i.e. software, cartridge, and phonoamp) and downstream (interconnects and amplifier). If you are going through system changes (like I am at the present), or you play through a variety of sources, I can see where a passive might not be flexible enough. And, of course, it also depends on how revealing your system is.
I have not yet tried a transformer-based passive (I've got one coming) and I'm not finished testing and developing yet, but the fact that a DIY set of resistors, 24-position switch, and aluminum box with RCA connectors for about $400 is giving big-name linestages (for >20x the $'s) a run for their money should say something!
Like it's really worth taking the time to investigate whether it's going to work for you in your system.